1. Field
The present application relates to switches. More particularly, the present application relates to an antenna transmit receive switch.
2. Description of Related Art
A wireless device such as a cellular phone will typically use one antenna for both transmitting and receiving. During periods of time when the wireless device is receiving, one or more switches in the wireless device can be utilized to connect the antenna to receive circuitry and disconnect the antenna from transmit circuitry. During periods of time when the wireless device is transmitting, the switches can be utilized to connect the antenna to transmit circuitry and disconnect the antenna from receive circuitry. The receive circuitry, transmit circuitry, and antenna normally do not all share the same impedance value.
Generally, when connecting two circuit elements with different impedance values, impedance matching networks are used to transform an impedance of a first circuit element to an impedance of a second circuit element in order to reduce or eliminate power loss due to reflection and optimize power transfer between the first circuit element and the second circuit element. Consequently, in the exemplary case of a wireless device, impedances of the receive circuitry, transmit circuitry, and antenna usually are transformed to a common system impedance when connected via the switches.
A commonly used value for system impedance in wireless design is 50Ω. Consequently, as an example, consider an impedance matching network that is adapted to transform an impedance to the system impedance of 50Ω. Receive circuitry typically has an impedance close to or equal to the system impedance of 50Ω, resulting in little or no power loss in the impedance matching network. The transmit circuitry and antenna usually have low impedance values relative to the system impedance of 50Ω. For example, a power amplifier of the transmit circuitry can have an output impedance of 2Ω whereas an antenna can have an impedance of 10Ω to 20Ω. Impedance transformation from such impedance values to 50Ω can result in nontrivial power loss. It is noted that each of the transmit circuitry, receive circuitry, and antenna can also contain a reactive component in its impedance.